The present invention relates generally to enclosures. More specifically the present invention relates to molded plastic enclosures for electrical equipment.
Non-automatic (manual) electrical disconnects are located near electrical equipment such as motors, compressors, motor controls or other electrically driven machinery. Electrical disconnects allow maintenance personnel to manually isolate the machinery from electrical power when maintenance or replacement of the machinery is necessary, thus protecting the personnel from electric shock.
Industrial air conditioning and refrigeration equipment, and the disconnects that service this equipment, are often located on roof tops or other outdoor locations. Consequently, the electrical disconnects must not only protect personnel from the potential hazards of electric shock, but must also be water-resistant and tamper-proof.
Electrical disconnects generally comprise a disconnect switch assembly (switch) and related connecting equipment, e.g., lugs and ground terminals, housed within a disconnect enclosure (enclosure). When the disconnect is installed, the switch is electrically connected to both the power supply (line) wiring and the equipment (load) wiring via the lugs. Thus, the switch forms part of the electrical circuit providing electrical power to the electrically driven machinery.
The enclosure is generally shaped as an elongated parallelepiped, with a top wall, a bottom wall, a back wall, two side walls, and a cover. The cover allows access to the switch and related connecting equipment housed within the enclosure. The enclosure is generally constructed of plastic or metal, and it must meet a variety of accepted industry standards, including Underwriters' Laboratory (UL) standards or National Electric Code standards, depending on their use. One such standard is UL 50, entitled "Standard for Safety for Enclosures for Electrical Equipment."
A water-resistant metal enclosure of the prior art comprises a cover hingedly attached to the side walls near the top wall, such that the cover pivots upwardly to open. Water resistance is provided by a rain shield formed on the top wall, which extends downwardly over the top portion of the cover when the cover is in the closed position. Water resistance is further provided by rims extending from the periphery of the cover, which extend over a portion of the side and bottom walls when the cover is in the closed position. Unfortunately, the rain shield interferes with the opening of the cover. In addition, the cover, which pivots upwardly to open, must be held open by some mechanical means to allow the electrician to freely access the internal components.
These problems are overcome in the prior art by the slotted arrangement of the hinge attaching the cover to the side walls. The hinge comprises slots disposed in opposing rims of the cover, which accept dimples protruding outwardly from the upper end of each side wall. The slots allow the cover to slide downwardly to avoid the rain shield. The cover can then be pivoted upwardly to the open position. Once in the open position, the slots allow the cover to be pushed towards the side walls, where the cover is received by slots disposed in the side walls. The slots in the side walls hold the cover in an open position. Unfortunately, the manufacture and assembly of the complex metal cover and rain shield is time consuming and labor intensive. Additionally proper production tooling, e.g., welding equipment, must be used and maintained during the assembly process.
Enclosures for electrical disconnects further comprise a plurality of knockouts, which are generally located near the lower sections of the enclosure. The knockouts allow the electrician installing the disconnect to create circular access holes of predetermined diameters in the housing. The access holes allow the line and load wiring to pass into the enclosure. In addition, the access holes allow electrical conduit, which houses the line and load wiring, to be secured to the housing by standard connecting hardware. Generally, only two access holes are used, one for the line wiring and one for the load wiring. However, to allow flexibility in the installation of the disconnect, there are usually more than two knockouts in the enclosure.
The knockouts comprise a plurality of concentric circular sections formed by circular scores in the walls of the enclosure. Each section has a predetermined outside diameter corresponding to the diameter of standard electrical conduits. The scores in the enclosure allow the electrician installing the disconnect to remove, or knock out, the appropriate number of sections so that the diameter of the access hole corresponds to the size of electrical conduit being used.
It is important that the access hole formed by the knockout is properly fit to the mating conduit. Therefore, UL Standard 50 requires that the concentric sections of the knockout are capable of being removed (knocked out) one at a time starting with the smallest-diameter section and consecutively moving up to the larger-diameter sections. If multiple circular sections of the knockout are inadvertently removed during installation, the access hole could be too large for the conduit. Consequently, time-consuming adjustments would have to be made to fit the conduit to the enclosure.
Knockouts in metal enclosures of the prior art are formed during the manufacture of the enclosure by stamping the circular scores, which define the circumference of the concentric sections, into the enclosure. Discontinuities in each of the circular scores create a plurality of tabs extending radially across the concentric sections. To remove a section, the electrician breaks the tabs for that particular section. The remaining tabs provide the strength to hold the remaining sections in place. Metal enclosures of the prior art typically have three concentric sections to mate with three standard sizes of conduit.
Knockouts in plastic enclosures of the prior art are formed by molding the circular scores, which define the circumference of the concentric sections, into the enclosure. However, the use of knockouts in plastic enclosures presents a number of problems. First, plastic knockouts are inherently more difficult to remove than metal knockouts due to differences in material properties such as ductility. Consequently, the scores must be deep enough to allow easy removal of each section, but not so deep as to destroy the structural integrity of the non-removed sections. Second, the plastic knockouts must meet the requirement of UL Standard 50. That is, the concentric sections of the knockout must be capable of being removed one at a time. To meet this requirement, the depth of the circular score for smaller-radius sections must be greater than the depth of the circular score for larger-radius sections. In other words, for a pair of concentric sections, the circular score at the circumference of the inner section is deeper than the circular score at the circumference of the outer section, allowing the inner section to be more easily removed. Unfortunately, the aforementioned problems limit a plastic enclosure to having a knockout of only two concentric sections. The use of a greater number of concentric sections requires either that the largest-diameter section has a circular score of such shallow depth that the section can't be easily removed, or that the smallest-diameter section has a circular score so deep that the non-removed sections will have no structural integrity.